The present invention relates to an oil pressure control system for an automatic transmission and, more particularly, to an oil pressure control system for a shift-by-wire automatic transmission for changing the running ranges on the basis of electric signals.
As is well known in the art, an automatic transmission for automobiles Is constructed to execute gear changes by changing the engaged/disengaged states of frictional engagement elements such as clutches and brakes with oil pressures. The decisions of the gear changes are generally accomplished on the basis of an engine load represented by a throttle opening and a vehicle speed. These shifting operations based on the running condition are performed for a plurality of forward gear stages. The selection of the running ranges such as a running range for an engine braking at middle or low gear stages, a reverse range or a neutral range is generally carried out in the prior art by operating a shift lever manually.
On the other hand, there has been proposed in recent years an automatic transmission which is constructed to select the running ranges electrically by switching actions. According to this construction, valves for switching the running ranges are actuated by energizing predetermined actuators by the switching actions. The system of this kind is generally equipped with fail-safe means, i.e., means for coping with failures in the electric system or the valve mechanism. For example, Japanese Patent Laid-Open No. 184249/1987 discloses a system equipped with an emergency valve. This system, as disclosed, is constructed to execute the gear changes by operating a rotary valve with a motor. The emergency valve is disposed in parallel with that rotary valve so that the oil pressure may be fed through the emergency valve to predetermined frictional engagement elements by blocking the feed of the oil pressure to the rotary valve if the rotary valve is troubled.
The system having the aforementioned emergency valve of the prior art is enabled to set predetermined one of gear stages by engaging the predetermined frictional engagement elements fed with the oil pressure through the emergency valve, even if the rotary valve for executing the gear changes fails to operate normally. However, the gear stage to be thus set is limited to the predetermined one of the forward gear stages so that the system cannot always allow the running according to the various changing situations such as the road condition even if it can effect the forward running. Thus, the system of the prior art is inferior in utility.
On the other hand, Japanese Patent Publication No. 25505/1977 discloses a system for deciding a failure in the control lines in terms of the logic between detected oil pressures and electric signals so that a safe state may be maintained by stopping the engine if the failure takes place.
According to the system described above, the vehicle itself is stopped, if the control line is troubled, so that the safety can be negatively maintained. With the engine being stopped, however, the vehicle cannot run any more. As a result, the vehicle can neither relieve itself positively to a safer state nor use a still valid control line, if any, effectively for a continuous run. Thus, this system cannot always be said to be sufficient either for the so-called "positive fail-safe".